More particularly, the embodiments of the invention relate to a writing implement that includes a deformable hollow body extended along a central axis as far as an end, and an end member that includes a tubular portion which has an inner face with a plurality of first identical projecting reliefs suitable for engaging with and radially deforming the hollow body during the fitting of the end member by engagement of its tubular portion around the end of the hollow body.
The fitting of such an end member must satisfy two types of constraints involving sometimes incompatible solutions, namely on the one hand simplicity of manufacture, making it possible to reduce the overall cost of production, and on the other hand, a sufficiently robust assembly to avoid the user experiencing inopportune detachment of the end member.
It is desirable that the end piece cannot become detached unless an abnormally high pulling force is applied. This is in order to avoid the loss of the end piece which could detract from the appearance of the implement and its functional characteristics, or even result in stains in the case of a ballpoint or felt tip implement. In the particular case of a mechanical pencil, the end member often constitutes a holder receiving an interchangeable eraser, topped by a protective end cap. In addition, this assembly acts as a rear push-button. The fitting of the eraser holder onto the lead container must withstand the pulling forces applied repeatedly during removal of the eraser end cap and extraction of the eraser from its holder.
Moreover, for production, it is advantageous that the end member has shapes corresponding to a mold that is simple to machine, but above all that this member can reliably be removed from the mold at high speeds. Next, it is also preferable that the assembly can be produced rapidly, in particular in an automated process, taking account of the inevitable dimensional dispersions of the hollow bodies and end members produced. In particular, in the case of a mechanical pencil, the lead container tube constituting the hollow member is generally a tube made of extruded polypropylene produced by the meter and cut to length. It can have relatively large variations in outside diameter, inside diameter and radial elasticity. Its substantial length means that it is also relatively sensitive to buckling when a pressure is applied to the free rear end of the latter.
A tubular portion which has inner reliefs, in particular in the form of longitudinal ribs, can be molded easily. In order to reach a compromise between ease of fitting and the required gripping force, it is known to vary the height and the number of these projecting reliefs. But it is found that the number and the height of the reliefs rapidly reach a limit beyond which it is difficult to completely slip fit the end member. In addition, it is found that even if an adequate retaining force is obtained for exact given dimensions of the hollow body and of the end member, this force reduces very substantially in the case of variations in the dimensions, even if these variations remain within ranges that are usual in industrial production.
Moreover, from document JP10-203076 A a solution is known to the problem of the grip of an eraser holder on a lead container which consists of providing a protruding central portion which is inserted inside the lead container tube. But here again, the fitting option and the retaining force are susceptible to the variations of the inner diameter of the lead container tube. In addition, this type of solution makes it almost impossible in practice to refill the lead container by inserting the leads through a hole in the eraser holder.
There is therefore a need in the field of writing implements to improve the fitting of an end piece onto a hollow body, in particular in order to offer a compromise between ease of production and the quality of the grip for the user, while still tolerating some variability in the geometrical dimensions.